Receiver for frequency-modulated waves



Aug. 22, 1939; K, K PFMULLER 2,170,202

RECEIVER FOR FREQUENGY-MODULATED WAVES Filed Jan. 12, 1959 mvgmoa, IKARL KUPFMULLER MUM,

ATTORN EY Patented Aug. 22, 1939 RECEIVER FOR FREQUENCY-MODULATED WAVESKarl Kiipfmiiller, Berlin-Charlottenburg, Germany, assignor to FidesGesellschaft fiir die Verwaltung und Verwertung von gewerblichenSchutzrechten mit'beschrankter Haftung, Berlin, Germany, a corporationof Germany Application January 12, 1939, Serial No. 250,520 In GermanyJanuary 3, 1938 3 Claims.

In signal transmission on frequency modulated Waves, as is well known inpractice, one slope of the resonance curve is used for demodulation,with the result that the frequency variations are reconverted intoamplitude modulations, the current after rectification or detectionresulting in the AF modulation. In order that in frequency modulationthe relation between the current and the frequency variation may beunequivocal, the Width of the slope of the resonance curve must at leastbe equal to the aggregate frequency variation; for this reason, also thewidth of the resonance curve measured between the two half values, mustbe at least equal to this frequency range.

'For instance, for ultra-short waves this situation results in verybroad curves inasmuch as in this case, speaking in absolute values, verylarge frequency shifts are involved. For example, for a wave of 50 cm.,in the presence of a plate voltage variation by modulation of a fewpercent, the frequency variation often amounts to over 100 -kc.; hence,also one-half of the resonance curve must be at least of this width.

Now, the present invention is predicated upon this fact that the noiselevel due to (tube) noises of the receiver is a function of the width ofthe resonance curve. It is known that the noise voltage of a receiverwhich permits of the constant transmission of a frequency band havingwidth A is proportional to /Af.'

This noise may be considered as consisting of beat frequencies betweennoise voltages lying within the radio-frequency acceptance band. Ofcourse, only such beat frequencies as fall within the audio-frequencyband of the receiver are passed by the audio-frequency system. It iseasily seen that, in general, the total noise at the receiver outputwill be greater as the radiofrequency and audio-frequency pass bands arelarger, the first because more noise components are admitted, and thesecond because a wider range of beat frequencies can pass theaudiofrequency system.

In case, however, the radio-frequency band passed by the receiver filteris narrower than the audio-frequency band, the highest beat frequencythat can be formed between components that pass the radio-frequencyfilter is equal to the radio-frequency band width, so that it would makeno difierenceif the audio-frequency band were much larger. Hence, incase the radiofrequency acceptance band is narrower than theaudio-frequency band we have the situation that the total noise dependsonly on the radio-frequency band Width.

Now, according to the invention a receiver of lower noise voltage levelapplicable to frequency.- modulated waves is created in that the widthof the receiver filter is small compared with the aggregate frequencyvariation, while an unequivocal relationship between the outputpotential or current and frequency is established by a feedback. By theuse of a narrow frequency curve the noise voltage is lessened. However,the use thereof becomes possible only by virtue of the feedback sincewith a narrow resonance curve the existing frequency range would be, incontrast,.say, 300 kc.

By regenerating, for instance, part of the rectifier output, theresonance frequency of the receiver circuit could be re-set as afunction of the output potential or current in the same sense as theincoming frequency. Re-setting of the frequency, in superheterodynereceivers could also be effected by-controlling'the resonance curve ofthe IF from the rectifier output in accordance with the frequencyvariation of the transmitter so that as the frequency increases, thereis an increase in the resonance frequency of the oscillatory circuit orcircuits in the I. F. section, and vice versa.

'Insteadof shifting the resonance point of the I. F., with theoscillator frequency assumed to be stable, re-adjustment of thefrequency could be accomplished also by keeping the resonance point ofthe I. F. stable, while the oscillator frequency is subjected to controlby the rectified current in such a way that with growth of thetransmitter frequency, there occurs an increase in the frequency of theoscillator circuit. This control action could be effected, for instance,by causing the rectified current to alter the plate potential of theoscillator and incidentally its frequency.

In describing my invention in detail reference will be made to theattached drawing wherein Figure 1 is a resonance curve of a frequencymodulated wave receiver using circuits covering a considerable band offrequencies. Figure 2 is the resonance curve of a frequency modulatedreceiver in which by the use of my invention the resonancecharacteristic has been narrowed, while; Figure 3 illustrates a receiverarranged in accordance with my invention and includes means for shiftingthe point of operation of the receiver up and down the sharp resonancecharacteristic thereof.

Referring to the drawing, Fig. 1, A indicates the resonance curve of areceiver designed for frequency-modulated waves. The frequency range Fextends from )i to f2. For demodulation is used the slope of theresonance curve between f1 and f2. The operating point P shifts, as willbe seen, along the slope of the resonance curve between f1 and f2.

Fig. 2 by way of example shows a narrow resonance curve B of thereceiver, that is, a curve as used according to the disclosure with aview of securing a low noise voltage. The resonance curve B is plottedfor a frequency fx lying between f1 and f2. Inasmuch as the resonancecurve of the. receiver is changed'in the same sense as the transmitterfrequency, the operation point. P shifts along the workingcharacteristic B, shown by broken lines, across the family of resonancecurves rather than along the slope of the resonance curve properlyso-called. This shift of the resonance curve occurs at AF rate; hence,the building-up periods can be kept sufiiciently small.

By way of example, the principle hereinbefore described may be carriedinto practice by shifting the resonance curve, for instance, by causingthe rectified current to alter the biasing magnetization (magnetic bias)and thereby the inductance of the oscillatory circuit in a way asillustrated in Fig. 3.

' Fig. 3 illustrates a conventional type of heterodyne receiver forfrequency-modulated waves in which the object of the present inventionhas been incorporated by shifting the resonance curve of the I. F. Theoutput of the oscillator O and of the antennae are fed to the mixer tubeMR. After the mixing stage follows an I. F. stage ZV, the rectifier G1and the AF stage NFV. The rectifier G1 has in its output circuit a.resistance R1 in which a potential is produced by rectification of theamplitude variations on the intermediate-frequency energy passed bycircuits I and 2. These amplitude variations are produced due to thesloping characteristics of the circuits and correspond to frequencyvariations on the received wave. The oscillatory circuit l comprisinginductance L1 and condenser C1 and the oscillatory circuit 2 comprisinginductanc L2 and condenser C2 according to this invention have a narrowresonance curve. The resonance curve of the oscillatory circuits isshifted by tapping developed in said impedance.

part of the rectifier output voltage across R and this is used toregulate or control the biasing or tuning coils L1. and L2, if desired,after the AF components have been filtered out. By the change of thebiasing magnetization the inductance of the oscillatory circuit ischanged. The circuit containing the coils causing a change in-the biasmay also include a biasing or polarizing voltage battery B2.

What is claimed is:

1. In a frequency modulated wave receiver, means for heterodyning afrequency modulated wave to a lower frequency, an intermediate frequencyamplifier of a relatively narrow band width coup-led to said firstmeans, detecting means coupled to said amplifier and means controlled bypotentials derived from said detectin means for varying the meanfrequency of response of said intermediate frequency amplifier inaccordance with potentials derived from said detecting means. I

2. The method of demodulating frequency modulated wave energy andreducing noise components in the demodulated output which includes thesteps of amplifying said frequency modulated wave energy in an amplifierhaving a relatively narrow band width, and varying the position of saidband width in the frequency spectrum in accordance with frequencymodulations on said Wave energy.

3. In a frequency modulated wave receiver, means for heterodyning afrequency modulated wave to wave energy of lower frequency, an electrondischarge tube amplifier having input and output circuits, said circuitshaving a resonance characteristic of a width less than the Width of saidfrequency modulated wave of lower frequency, means for impressing saidWave energy of lower frequency on said input circuit, a rectifier havingan input coupled to said output circuit said rectifier having an outputincluding an impedance wherein potentials characteristic of thefrequency modulations on the wave are developed, and means coupled tosaid impedance and associated with said input and output circuits forvarying the tuning of said circuits in accordance with the amplitude ofthe potentials KARL

